Liquid nitrogen is commonly used by dermatologists and other medical practitioners for the treatment of various tumors and lesions. Such treatment is well accepted and has been utilized for a number of years. The methods of application vary widely. Some physicians, for example, apply the liquid nitrogen with a cotton swab. Others utilize a more sophisticated approach, using specially designed self-pressurizing flasks such as those shown in U.S. Pat. Nos. 3,702,114 and 3,739,956. The latter method, while effective, is somewhat limited by the relatively small volume of liquid nitrogen which can be contained in the flask. Furthermore, the self-pressurizing nature of the flask means that pressure drops considerably upon use so that sustained dispensing of the liquid nitrogen cannot be achieved.
In many larger liquid nitrogen systems, such as those for use in neurosurgery, the pressure problem has been overcome by the insertion of heaters into the liquid nitrogen containers. This has the obvious disadvantage of requiring an external power supply and also of introducing additional electrical circuitry into the operating room atmosphere. Still another problem encountered with these larger capacity liquid nitrogen systems, is that the valves controlling the liquid nitrogen flow are highly susceptible to freezing and become inoperable.
There are also certain circumstances wherein it would be highly desirable to be able to transport a liquid nitrogen treatment facility into the field. For example, liquid nitrogen has been used quite successfully by veterinarians for the treatment of certain diseases of cattle. To be successfully used for this purpose, the apparatus should be light in weight, of relatively high capacity, and not require any external power supply.
Other features, which are difficult to achieve in prior art systems but are highly desirable, are simple regulation of the liquid nitrogen discharge velocity, accurate control with abrupt termination of liquid nitrogen flow when desired, and a pulsed spray rather than a continuous stream. A pulsed spray reduces dripping of sprayed liquid and improves visibility at the site.
Accordingly, it is a primary object of the present invention to provide a simplified system for dispensing a liquid cryogen. Other objects are to provide such a system: which does not require an electrical power supply; wherein valving is not exposed to liquid cryogen; which has a large capacity of cryogen; which has sustained dispensing ability; which permits either continuous flow or pulsed flow; and which has improved accuracy of control. The manner in which these objects are achieved will become apparent from the following description and appended claims.